Within the semiconductor industry, lithographic printers with reticles (also called masks) having device patterns have been used to pattern photoresist layers for several years. State of the an semiconductor devices require very small dimensional patterns. The patterns may be formed within a photoresist layer, as long as a reticle pattern can be resolved within the photoresist layer. The resolution limit, which is herein defined as the smallest dimension that can be resolved within the photoresist layer while maintaining an acceptable process window, is about: ##EQU1## where k.sub.1 is a "constant" for a given lithographic process (process constant), .lambda. is the wavelength of the radiation, and NA is the numerical aperture of the lens. One skilled in the an appreciates that k.sub.1 is not a true constant, but can actually vary. A conventional reticle has chrome elements and openings between the chrome elements. A conventional reticle has a k.sub.1 of about 0.8. The resolution limit using a conventional reticle is hereinafter called the conventional resolution limit and is about 0.8 .lambda./NA. When .lambda. is about 365 nm and NA is about 0.54, the conventional resolution limit is about 0.54 .mu.m.
A phase-shifting reticle may include chrome elements, phase-shifting elements, and reticle openings, which are areas between both the chrome elements and the phase-shifting elements. A typical prior m phase-shifting reticle is shown in FIG. 2 and includes a chrome element 11, a phase-shifting rim 12, and a reticle opening 13. A phase-shifting rim is a type of phase-shifting element. The reticle is discussed in more detail below. The transmittance of radiation transmitted through the phase-shifting rim 12 is about the same as the transmittance of radiation transmitted through the reticle opening 13.
Within this application, a phase-shifting element width is sometimes expressed as a fraction of IRF.multidot..lambda./NA where IRF is the image reduction factor of the lens, .lambda. is the wavelength of the radiation, and NA is the numerical aperture of the lens. Widths expressed in units of IRF.multidot..lambda./NA are used because the actual width of a phase-shifting element varies based on those three parameters. The process constant, k.sub.1, decreases as the width of the phase-shifting element increases. Although k.sub.1 is a function of the phase-shifting element width, an equation to determine k.sub.1 given a phase-shifting element width is not known. FIG. 1 generally illustrates that k.sub.1 decreases as the phase-shifting element width increases, but it is understood that k.sub.1 may not be a linear function of the phase-shifting element width. When the prior art phase-shifting reticle has a phase-shifting element with a width greater than about 0.4 IRF.multidot..lambda./NA, the phase-shifting element is too wide, and the photoresist layer under the center of the phase-shifting element is substantially exposed when the reticle is exposed to radiation. The phase-shifting element width is usually no less than about 0.1 IRF.multidot..lambda./NA because k.sub.1 for a phase-shifting element width less than 0.1 IRF.multidot..lambda./NA is close to the same value as k.sub.1 for the conventional reticle. If the phase-shifting element width is about 0.4 IRF.multidot..lambda./NA, k.sub.1 is about 0.7, and the resolution limit is about 0.47 .mu.m when .lambda. is about 365 nm, and NA is about 0.54. The resolution limit is about 13% less than the conventional resolution limit for the same .lambda. and NA.
As the phase-shifting element width increases, at least one process complication occurs. The radiation intensity at the photoresist layer surface under the center of the phase-shifting element increases as the phase-shifting element width increases as shown in FIG. 1. As used in this patent application hereinafter, I is the radiation intensity at a point on a resist layer surface beneath the reticle, and I.sub.O is the radiation intensity incident onto the reticle when the reticle is exposed to radiation. I/I.sub.O in FIG. 1 is the ratio of intensities beneath the center of the phase-shifting element.
FIG. 2 shows a prior art phase-shifting reticle having a phase-shifting rim. The reticle comprises a reticle base including a quartz plate 10, a phase-shifting rim 12, and a chrome element 11. The phase-shifting rim 12 and chrome element 11 are in contact with the quartz plate 10. The reticle has a reticle opening 13, which is an area of the reticle where the quartz plate 10 does not have the phase-shifting rim 12 and the chrome element 11. The reticle is configured so that the reticle opening 13 is surrounded by the phase-shifting rim 12 that is surrounded by the chrome element 11. The phase-shifting rim 12 has a width of about 0.4 IRF.multidot..lambda./NA and a thickness such that radiation transmitted through the phase-shifting rim 12 is shifted about 180.degree. out of phase relative to the radiation transmitted through the reticle opening 13. The transmittance of radiation transmitted through the reticle opening 13 is about the same as the transmittance of radiation transmitted through the phase-shifting rim 12.
The reticle is used to pattern a photoresist layer as illustrated in FIGS. 3A, 3B, and 3C. FIG. 3A includes a cross-sectional view of the reticle in FIG. 2 and has the quartz plate 10, the chrome element 11, the phase-shifting rim 12, and the reticle opening 13. When radiation is incident on the reticle, the radiation is transmitted through the reticle opening 13 and the phase-shifting rim 12, but the chrome element 11 prevents virtually all transmission of radiation. FIG. 3B illustrates how I/I.sub.O may vary across the photoresist layer surface when using the reticle illustrated in FIG. 2. As seen with FIG. 3B, I/I.sub.O under the chrome element 11 is substantially zero, and I/I.sub.O under reticle opening 13 away from the phase-shifting rim 12 is close to unity.
Interference areas A21 and A22 each include a portion of the phase-shifting rim 12 as shown in FIG. 3A. Within each interference area, the radiation transmitted through the phase-shifting rim 12 is shifted about 180.degree. out of phase relative to the radiation transmitted through the reticle opening 13. Radiation from the reticle opening 13 that enters the interference areas is interfered with by the radiation that is transmitted through the phase-shifting rim 12 within the interference areas. In theory, the interference should prevent radiation from reaching the photoresist layer under the interference areas. After developing, a theoretical patterned photoresist layer has photoresist elements having a wall angle (.theta.) of 90.degree. without a recession and a photoresist layer opening having no residual photoresist layer. As used in this application, the wall angle is an angle formed by the edge of a photoresist element at a photoresist layer opening with respect to the substrate surface under the photoresist element. The wall angle is measured from the substrate surface, and a wall angle of 90.degree. is a vertical edge.
The actual patterned photoresist layer typically has at least one problem. As seen in FIG. 1, some radiation reaches the photoresist layer under a phase-shifting element regardless of the phase-shifting element width. FIG. 1 illustrates that I/I.sub.O beneath the center of the phase-shifting element increases as the width of the phase-shifting element increases. I/I.sub.O under the center of the phase-shifting element is about 0.15 when the phase-shifting element width is about 0.4 IRF.multidot..lambda./NA. After developing, the photoresist layer has resist elements 21 each with a recession 23 near a photoresist layer opening 22 as shown in FIG. 3C. The recessions have a depth greater than 10% of the resist element thickness away from the recession. When the phase-shifting rim 12 is relatively narrow (about 0.1 IRF.multidot..lambda./NA), the photoresist elements may have wall angles (.theta.) less than 80.degree. or greater than 100.degree. and are similar to photoresist elements formed using a conventional reticle to pattern a photoresist layer having a dimension smaller than the conventional resolution limit. A recession with a depth greater than 10% of the resist element thickness away from the recession or a wall angle less than 80.degree. or greater than 100.degree. may cause processing complications during subsequent processing steps including etching and ion implantation.
At an intermediate phase-shifting element width (between 0.1 and 0.4 IRF.multidot..lambda./NA), the resolution may be too low and the recession may be too deep. In determining the width of a phase-shifting element, one typically chooses the widest phase-shifting element width without giving a recession too deep within the photoresist element. Typically, the prior art phase-shifting element width is about 0.15 IRF.multidot..lambda./NA. At 0.15 IRF.multidot..lambda./NA, k.sub.1 is closer to a conventional reticle's k.sub.1 than k.sub.1 for a phase-shifting element width of 0.4 IRF.multidot..lambda./NA. Therefore, the prior art phase-shifting reticle only gives a marginal improvement in resolution.
Even if the reticle pattern may be resolved within the photoresist layer when the phase-shifting element width is about 0.15 IRF.multidot..lambda./NA, the process window for forming the photoresist pattern is smaller than when the phase-shifting element is closer to 0.40 IRF.multidot..lambda./NA. Again, there is a tradeoff. The tradeoff is between production repeatability and the depth of the recession.
The prior art phase-shifting reticle is difficult to generate. State of the art reticle generating equipment currently has a resolution limit slightly below 1 .mu.m. A prior art phase-shifting element typically has a width of about 0.15 IRF.multidot..lambda./NA, which is about 0.51 .mu.m wide when IRF is about 5, .lambda. is about 365 nm, and NA is about 0.54. Therefore, a reticle having a phase-shifting element with a width of about 0.51 .mu.m is difficult to generate.
The previous discussion focuses on a positive photoresist layer. A negative photoresist layer has a reversed image compared to the positive photoresist layer. After developing, a negative photoresist layer formed using the prior art phase-shifting reticle has a negative photoresist element under the reticle opening and negative photoresist layer openings under phase-shifting rim and the chrome element. The negative photoresist element has a wall angle less than 80.degree. or greater than 100.degree., or each negative photoresist layer opening has a residual negative photoresist layer with a thickness greater than 5% of the negative photoresist element thickness. The negative and positive photoresist layers formed using the conventional reticle or the prior an phase-shifting reticle may cause complications during subsequent processing steps.